How to solve Zoho Project API > General error 6500 ?

How to solve Zoho Project API > General error 6500 ?

Hello ! I am working on Zoho Projects API. I have  got a Ticket id when sending an http post. Also I got an API Key
from zoho...Now I am sending another http post request to get user portals. However I got the ff. respond...
      <error><code>6500</code><message>General error</message></error>
Is there a problem with my API key? How can I know that API key is valid?
Anybody may help? please !